1. Field of the Invention
The present invention relates to a rack-assist type electric power steering apparatus, and more particularly to a rack-assist type electric power steering apparatus, in which when a ball nut slides a rack bar while rotating, a vibration and a noise in the axis direction and the radial direction generated from the rack bar and a rack housing are absorbed, thereby facilitating the operation of the ball nut and the rack bar, reducing the vibration and the noise, and providing a comfortable steering feeling to a driver.
2. Description of the Prior Art
In general, as an electric power steering apparatus for a vehicle, a hydraulic power steering apparatus using oil pressure of a hydraulic pump is used. However, an electric power steering apparatus using a motor has recently gradually been universalized.
A general electric power steering apparatus includes a steering system extending from a steering wheel to both side wheels 126, and an auxiliary power mechanism for supplying auxiliary steering power to the steering system.
The auxiliary power mechanism includes a torque sensor, an electronic control unit (ECU), a motor, and a belt-type transmission device. The torque sensor senses a steering torque applied to the steering wheel by a driver, and outputs an electrical signal in proportion to the sensed steering torque. The ECU generates a control signal based on the electrical signal transferred from the torque sensor. The motor generates auxiliary steering power based on the control signal transferred from the ECU. The belt-type transmission device transfers the auxiliary power generated by the motor to a rack bar through a belt.
FIG. 1 is a partial cross-sectional view schematically illustrating a conventional rack-assist type electric power steering apparatus, and FIG. 2 is a cross-sectional view schematically illustrating a part of a conventional rack-assist type electric power steering apparatus.
As shown in FIGS. 1 and 2, the conventional rack-assist type electric power steering apparatus includes a rack bar 140, a pinion shaft 110, a ball nut 210, a belt-type transmission device 160 and a motor 150. The rack bar 140 extends in the vertical direction of a vehicle, and is provided with a rack gear at one side of the outer circumferential surface thereof. The pinion shaft 110 is provided with a pinion gear engaged with the rack gear. The ball nut 210 is engaged with an outer circumferential screw thread 145 through a ball 220. The belt-type transmission device 160 connects the ball nut 210 to a motor shaft 155.
The pinion shaft 110 is connected to a steering wheel through a steering shaft, and the rack bar 140, provided with the outer circumferential screw thread 145 with a predetermined length at one side of the outer circumferential surface thereof, is embedded in a rack housing 120.
The belt-type transmission device 160 includes a belt connecting the ball nut 210 to the motor shaft 155, and transfers the auxiliary steering power generated by the motor 150 to the rack bar 140 through the ball nut 210 in proportion to the steering torque applied to the steering wheel.
As shown in FIG. 2, the conventional rack-assist type electric power steering apparatus includes the rack bar 140, the ball nut 210, the ball 220, an end cap 230, and the like. The rack bar 140 is provided with the outer circumferential screw thread 145 at the outer circumferential surface thereof. The ball nut 210 is provided with an intermediate path 215 for circulating the ball 220. The ball 220 rolls while coming in contact with the outer circumferential screw thread 145 of the rack bar, and the intermediate path 215. The end cap 230 is attached with the end of the ball nut 210.
In such a rack-assist type electric power steering apparatus, the ball 220 rolls while coming in contact with the outer circumferential screw thread 145 of the rack bar 140, and an inner circumferential screw thread of the ball nut 210, which slides the rack bar 140.
However, such a conventional rack-assist type electric power steering apparatus has a problem in that when the ball nut slides the rack bar by rotating, a noise and a vibration are generated through the rack bar and the rack housing. Furthermore, there is a problem in that such a noise and vibration are transferred to a driver together with other components, thereby reducing the steering feeling of the driver.